Status and Distribution of the Chesapeake Logperch Percina bimaculata Haldeman, 1844 in Pennsylvania

Abstract

The Chesapeake Logperch, Percina bimaculata, is a small fish endemic to the upper Chesapeake Bay drainage with a range significantly reduced by water quality and habitat degradation. It was described by Haldeman in 1842 and 1844 from the Susquehanna River, Pennsylvania, synonymized with the Logperch (Percina caprodes) by Jordan in 1877, and redescribed by Near in 2008. It is extirpated from the Potomac River drainage and extant in a portion of its historic range within the Susquehanna River drainage of Pennsylvania and Maryland in the United States of America. We reviewed available historic field survey information and defined the likely extent of the historic distribution of P. bimaculata and its change in response to historic environmental perturbations. We performed fisheries surveys during 2004–2023 to fill data gaps and to define the current distribution and abundance in the Susquehanna River drainage within Pennsylvania. Our surveys utilized electrofishing gears, electrified benthic trawls, and seines. Our surveys targeting P. bimaculata on the Susquehanna River and tributaries from York Haven to Holtwood, Pennsylvania, combined with available fisheries surveys conducted by others, confirm it was extirpated from that portion of the drainage. We documented the continued usage of Conowingo Pond, Susquehanna River, and tributaries below Holtwood by populations known to exist in historic field surveys. These populations appear to have been stable since at least the 1960s based on historic data and our current surveys. We documented previously unknown populations in the Octoraro Creek drainage, Octoraro Reservoir, and Muddy Run Pumped Storage Facility. Habitat utilization observations during our surveys indicate that clean sand, gravel, rubble, boulders, and large woody debris are preferred substrates. In the lower Susquehanna River and two tributaries, P. bimaculata has demonstrated a plasticity of habitat usage by continuing to occur in environments converted into impoundments by dams. In these impoundments, shorelines and connections with clean tributaries containing preferred substrates appear to be important given their utilization by P. bimaculata during our surveys. It is our opinion that the Susquehanna River and tributaries above Holtwood once again constitute suitable habitats. Conservation efforts focused on reintroductions, water quality, habitat, and connectivity have the potential to increase the extent of occurrence, abundance, and security of P. bimaculata.

1. Introduction

The Chesapeake Logperch, Percina bimaculata (Etheostomatini Percidae), is a medium-to-large darter with a small global range limited to the lower Potomac and Susquehanna river basins (Figure 1). It was described by Haldeman [1] from the Susquehanna River, Pennsylvania initially as Perca (Percina) nebulosa. Haldeman later described P. bimaculata, also from the Susquehanna River, in a letter to D. H. Storer which was later published [2]. The two were subsequently synonymized with the Logperch, Percina caprodes [3,4,5], which ranges through the Great Lakes-St. Lawrence, Hudson Bay, and Mississippi River systems [6]. After the initial descriptions by Haldeman [1,2], P. bimaculata was extirpated from the Susquehanna River drainage above Holtwood, Pennsylvania. The population in the Potomac River drainage was last documented in the 1930s [7].

Near [8], using morphological and molecular data, concluded that the Chesapeake logperch is in fact a distinct species. He also suggested the most appropriate scientific name and describer to be Percina bimaculata Haldeman, 1844 since the original 1842 epithet was preoccupied by Perca nebulosa Rafinesque 1814 [9]. Near and Bernard [10] and Near [8] determined P. bimaculata to be more closely related to the Mobile Logperch (Percina kathae) and the Southern Logperch (Percina austroperca) but did not discuss the curious zoogeographic implications of these relationships. Later, Near et al. [11] reported P. bimaculata to be most closely related to P. caprodes. Jenkins et al. [12], Hocutt et al. [13], and Stauffer et al. [14] suggested the Chesapeake Bay drainage populations of P. bimaculata could have arisen from a faunal transfer of P. caprodes semifasciata via a northern glacial outlet during the Pleistocene. Stauffer et al. [14] also discuss morphological similarities and differences with other logperches.

The conservation status of P. bimaculata reflects its reduction in distribution and extant threats. In 2012, the Pennsylvania Fish and Boat Commission (PFBC) listed P. bimaculata as Pennsylvania Threatened under Chapter 75 of the Fish and Boat Code Title 58 PA Code [15]. Inclusion under Chapter 75 provides protection consideration in various state-level permitting processes. It is also listed as a state-threatened species in Maryland. In the 2015–2025 Pennsylvania Wildlife Action Plan [16], with species prioritization category values ranging from one (highest) to six (lowest), this species received a priority rank of one in the “contribute to the conservation of globally or regionally important species” category and a rank of two in the “maintain rare native species” category. It is also classified as G1G2 (global status) and S2 (subnational status) by NatureServe [17]. It is considered to be endangered by the American Fisheries Society Endangered Species Committee [18], is listed in the 2010 update of Threatened Fishes of the World [19], and is considered vulnerable in the IUCN Red List of Threatened Species [20]. The PFBC developed and has implemented a Chesapeake Logperch Species Action Plan [21]. Similarly, the United States Fish and Wildlife Service (USFWS) developed a conservation strategy for P. bimaculata with input by biologists and resource managers from Pennsylvania and Maryland [22].

The purpose of this paper is to report our review of relevant historic information and our status and distributional investigations in Pennsylvania. We define the likely extent of its historic distribution, its change in response to historic environmental perturbations, and its resulting current distribution and abundance. Understanding the historic changes and the current status of populations of P. bimaculata is important for effective conservation planning. Biologists at the PFBC, Maryland Department of Natural Resources (MDNR), and USFWS are currently working to implement their respective adaptive action plans for the recovery of P. bimaculata. The USFWS is currently reviewing P. bimaculata for potential federal listing, necessitating a full understanding of its status.

In 2018, the PFBC, MDNR, The Pennsylvania State University (PSU), Susquehanna River Basin Commission (SRBC), Conservation Fisheries Inc., and multiple elements of the USFWS initiated research and recovery efforts for P. bimaculata. These efforts have included life history research, population genetics, and reintroductions supported by the captive propagation and translocation of wild-caught fish. Successful reintroduction to the Susquehanna River drainage above Holtwood, Pennsylvania would increase the security of P. bimaculata, precluding the need for federal listing. Furthermore, it is hoped these efforts can inform potential conservation actions in the Potomac River drainage.

Review of Water Quality and Habitat Changes: The Susquehanna River is the largest river draining the Atlantic Slope entirely within the United States of America and it has a drainage area of 71,251 km² (27,510 mi²) [23]. In Pennsylvania, the Susquehanna River drains most of the central part of the state (Figure 2), and as it approaches its lower course, its valley narrows and deepens. The lowest 60 km (36 mi) of the Susquehanna River is known as the Conejohela Valley. The Conejohela Valley starts in the vicinity of Columbia and Wrightsville, Pennsylvania and descends steeply (approximately 2 m (6.5 ft) per river kilometer, [24]) to the tidally influenced “Susquehanna Flats” at the head of the Chesapeake Bay. The Fall Zone transition of the Piedmont Physiographic Province to the Coastal Plain extends upriver to the vicinity of Conewago Falls near Falmouth, Pennsylvania and includes the Conejohela Valley [25]. Habitats in this section of river and the lower reaches of many tributary streams were dominated by craggy dolomite, limestone, shale, quartzite, or schist rocks. In the river, these rocks take the form of bedrock shelfs, tilted ridges, massive boulders, rubble, gravel, and sand [26]. Flow conditions in the river were characterized by swift cascades, chutes, rapids, riffles, and runs, flanked by extensive wetlands in the floodplain. These conditions were particularly pronounced in the Conejohela Valley, and they made river transportation difficult, dangerous, and unpredictable [27]. Similar high gradient conditions also made for good mill dam sites on tributaries.

During the 18th and 19th centuries, timber, coal, iron, and agricultural goods were transported from throughout the upper Susquehanna River system to Columbia and Wrightsville (located across the river from each other). The logging industry used the river and its tributaries to float log rafts to their destinations. The hazardous nature of travel on the river in the Conejohela Valley led to the eventual development of more reliable transportation around it. Rail and canal connections to Philadelphia and Baltimore led to Columbia and Wrightsville becoming the lowermost shipping hub on the Susquehanna River in Pennsylvania. Many mills and iron furnaces were located on tributaries to the river, where dams were constructed for waterpower and access to the resources was made available due to river, canal, and eventually railroad transport.

The first major dam on the Susquehanna River was constructed between Columbia and Wrightsville circa 1835, just prior to the time of Haldeman’s descriptions [1,2]. The Columbia and Wrightsville Dam was 2.1–2.4 m (7–8 ft) in height and created a canal basin for the Eastern Division of the Pennsylvania Canal, Susquehanna and Tidewater Canal, and the Columbia–Philadelphia Railroad [28]. The construction of the Columbia and Wrightsville Dam also created one of the first major barriers to fish passage in the lower Susquehanna River drainage and created friction between fishermen and the dam owners [4,28,29]. This dam was breached in 1895 and 1896 and never rebuilt [30].

Starting in 1901, four major hydroelectric dams were constructed on the Susquehanna River that have acted as complete barriers to fish passage (listed here in descending order): York Haven Dam and East Channel Dam, 1904; Safe Harbor Dam, 1931; Holtwood Dam, 1910; and Conowingo Dam, 1928. Each of the hydroelectric dams is equipped with facilities designed to pass migratory fishes and operate seasonally from April through June. None of these fish passage initiatives were meant to facilitate the movement of small, nongame fishes such as P. bimaculata.

In addition to the four hydroelectric dams on the lower Susquehanna River, there are four large power generation stations, described here in descending order. Holtwood Steam Plant, a coal-fired steam electric generation facility, was built adjacent to Holtwood Dam in 1925 [31]. Brunner Island Steam Electric Station is a coal and natural gas fired electric generation plant located 2.25 km (1.4 mi) below York Haven Dam. The plant was developed during 1961–1969, and warm water effluent from cooling operations at Brunner Island affects the Susquehanna River [32,33]. Muddy Run Pumped Storage Facility is a hydroelectric power plant that opened in 1968 and is located 3.7 km (2.3 mi) downstream of Holtwood Dam. This station lifts water from the river through 4.6 m (15 ft) wide pipes into the reservoir above created by the damming of Mudd Run [34]. Peach Bottom Atomic Power Station is a nuclear power plant located 9.5 km (5.9 mi) below Holtwood Dam and was first opened in 1958. This power station releases warm water effluent from its cooling towers into the Susquehanna River, which influences the temperature regime in portions of Conowingo Pond [35,36].

The placement of these hydroelectric dams destroyed much of the original riverine habitat below Harrisburg, particularly in the Conejohela Valley. One report noted the presence of shoals and rapidly flowing habitats before the construction of Conowingo Dam [37]. Today, the vast majority of those shoals, chutes, cataracts, rapids, riffles, and runs are impounded and silted over.

Substrates located directly below the dams are generally representative of historic conditions noted in the Conejohela Valley; however, the Susquehanna River flows through them in a modified way. The site conditions below each dam can vary depending on river conditions and power generation operations. The river flow below Safe Harbor and Holtwood dams is concentrated along the left descending bank where the hydroelectric units are located, leaving the right descending bank with a comparatively lower flow. Lake Aldred, above Holtwood Dam, is susceptible to drops in dissolved oxygen and relatively higher temperatures in the summer [38]. The conditions below York Haven and East Channel dams provide more flow across the Susquehanna River, in part due to the leakage through the York Haven Dam, but also due to the presence of an island between these dames, necessitating releases on both sides of the river.

Like many other eastern rivers in the United States, the Susquehanna River was beset by serious water quality issues from the early 19th century through the 1970s. A state health official stated in 1937 that “The liquid between the river banks in most of our industrialized sections is no longer water in its strict sense, but a conglomeration of human excreta and industrial wastes” [27]. Multiple large population centers are present in the lower Susquehanna River drainage in Pennsylvania, including Harrisburg (state capital), Lancaster, York, Columbia, and Wrightsville. These cities are either located on the Susquehanna River or on tributaries draining to the river above Holtwood.

The impacts of coal extraction in the watershed were particularly problematic. A 1917 survey of the North Branch Susquehanna River notes coal silt 4.6–7.6 m (15–25 ft) deep in the river channels [27]. Coal fines were washed downriver and accumulated behind Holtwood Dam in such quantities that the Holtwood Steam Plant was constructed to augment the hydroelectric operation there, utilizing readily available river-dredged coal [31]. Coal dredging also occurred in Lake Clarke after Safe Harbor Dam impounded that section of the river. Between 1925 and 1948, 1 × 10⁶ tonnes (2 × 10⁶ tons) of anthracite coal had been recovered from silt-dredging operations dubbed the “Hard Coal Navy” [39]. The Holtwood Steam Plant utilized river-dredged coal until 1973. Coal fines are still readily observable at many locations on the lower Susquehanna River.

Present-day water quality and habitat issues in the lower Susquehanna River Basin result from mining, agriculture, municipal sources, industry, power generation, on-lot sewage, and urban runoff [40]. These activities have increased metal concentrations, the amount of suspended and dissolved solids, nutrient loading, acid rain, heated effluents, oxygen demand, pH levels, polychlorinated biphenyls (PCBs), pesticides/termiticides, and sedimentation [40,41,42,43,44]. Sediment transport is still a major issue in the lower Susquehanna River. Safe Harbor, Holtwood, and Conowingo dams and their impoundments capture 50% to 70% of the sediment headed toward the Chesapeake Bay [45]. Maintenance dredging of these sediments has been proposed to increase retention behind the dams and prevent further transport to the Chesapeake Bay.

Despite continued water quality and habitat challenges, today, the fish fauna of the lower Susquehanna River and tributary streams appear to be generally healthy [46]. Nutrient and sediment riverine loadings have declined over the past three decades in the river above Conowingo Pond [47,48]. Current fish assemblages in these pools and tributaries include four commonly distributed darter species: the Tessellated Darter (Etheostoma olmstedi) and Shield Darter (Percina peltata), both of which are native, and the Greenside Darter (E. blennioides) and Banded Darter (E. zonale), both of which are introduced [49,50].

2. Methods

We reviewed available information detailing historic (prior to 1950), recent historic (1950–2000), and recent (post 2000) fisheries surveys conducted within the range of P. bimaculata. These data were found in published studies in the literature, unpublished reports, or in the PFBC, PSU, and SRBC resource databases or files. Survey types and purposes that reported collections of P. bimaculata ranged widely and were rarely standard from collection to collection. Abundance reporting varied and consisted of relative abundance, coarse abundance, or presence–absence, or it was presented in summary form. A summary of data sources utilized is found in

Table 1. List of collectors contributing to known occurrences of Percina bimaculata in Pennsylvania during the historic (prior to 1950), recent historic (1950–2000), and recent (post 2000) periods. Map identification number corresponds with HUC 12 watersheds in Figure 3, Figure 4 and Figure 5. Pennsylvania Fish and Boat Commission (PFBC), The Pennsylvania State University (PSU), Susquehanna River Basin Commission (SRBC), Robert W. Criswell (RWC), Academy of Natural Sciences Philadelphia (ANSP), Virginia Polytechnic Institute and State University (VA Tech).

Map ID#	HUC 12 Name	Period	Surveys	Survey Totals	Collector
10	Susquehanna River—Fishing Creek	Historic	2	2	Samuel S. Haldeman
17	Susquehanna River—Muddy Creek	Recent Historic	1		Ichthyological Associates (JRS)
18	Muddy Creek	Recent Historic	1		Ichthyological Associates
19	Susquehanna River—Haines Branch	Recent Historic	30		Ichthyological Associates (JRS), Normandeau, PFBC, PSU, Radiation Management Corp.
20	Susquehanna River—Conowingo Dam	Recent Historic	7	39	Normandeau, PFBC
17	Susquehanna River—Muddy Creek	Recent	14		Normandeau, PFBC, PSU, RWC, SRBC
18	Muddy Creek	Recent	8		PFBC, PSU, RWC
19	Susquehanna River—Haines Branch	Recent	77		Lancaster Conservation Dist., Normandeau, PFBC, PSU, RWC, SRBC
20	Susquehanna River—Conowingo Dam	Recent	5		Normandeau, PFBC, PSU, RWC
21	East Branch Octoraro Creek	Recent	8		PFBC, PSU, RWC, VA Tech
22	West Branch Octoraro Creek	Recent	34		PFBC, PSU, RWC
23	Octoraro Creek	Recent	12	158	ANSP, Normandeau, PFBC, PSU
	All HUC12	All Periods		199	All Collectors

.

We conducted fisheries surveys during 2004–2023 to determine the current status and distribution of P. bimaculata. These data were supplemented with contemporaneous survey information from other collectors when available. Our collections are noted with our affiliations (PFBC, PSU, and SRBC) unless initials are used (J.R.S., R.W.C.) in Table 1. The focal area surveyed was the Susquehanna River drainage, including tributaries, downstream of York Haven Dam in York, Lancaster, and Chester counties—the likely historic extent of occurrence in Pennsylvania. A total of 199 surveys reported P. bimaculata, two in the historic period, 39 in the recent historic period, and 158 in the recent period (Table 1). Of those surveys, 119 were conducted by us in the recent historic (P.S.U., J.R.S.) and recent periods.

Our surveys primarily utilized backpack and boat electrofishing gears, but towed barge electrofishing gear, electrified benthic trawls, and seines were also used. Backpack and towed boat electrofishing gears and seines were used in streams and wadable sections of the Susquehanna River. Boat electrofishing gears and electrified benthic trawls were used to sample non-wadable sections of the Susquehanna River and tributary mouths. Electrofishing units used included Coffelt Electronics Co. (Englewood, CO, USA) BP-1C and CPS; Smith-Root Inc. (Vancouver, Washington, DC, USA) 12B, 15D, LR-20B, LR-24, 2.5GPP, VIa, and APEX; Electrofishing Systems LLC (ETS; Madison, WI, USA.) ABP-2 and MBS; and Midwest Lakes Electrofishing Systems (MLES; Polo, MS, USA) Infinity XStream. Our electrofishing surveys most commonly used pulsed direct current waveforms (30 Hz, 60 Hz, and 120 Hz at various duty cycles depending on the unit and its adjustability), in addition to limited usage of straight direct current (100% duty cycle) and gated burst waveforms. The waveform used was dependent on the unit available at the survey time and the intended outcome. Power output was adjusted according to ambient conditions and desired effect under those conditions, given the waveform used. Dip (scap) nets utilized during electrofishing surveys were constructed of 4.76 mm (3/16 in) mesh. Seines used were 3.05 m (10 ft) and 6.10 m (20 ft) in length with 4.76 mm (3/16 in) mesh. The electrified benthic trawl (Innovative Net Systems; Milton, LA, U.S.A.) used was 2.44 m (8 ft) wide at the mouth and was constructed of 4.76 mm (3/16 in) mesh. This trawl was custom-fitted with electrodes at the PFBC and paired with a Smith-Root Inc. VIa or APEX electrofishing pulsator powered by a Honda Power Equipment Mfg., LLC (Torrance, CA, U.S.A.) 3500-watt generator.

Our reporting of distributional information is written in a manner that we deem sufficiently detailed to be informative, as well as protective of sensitive populations that are currently listed as Pennsylvania Threatened. To accomplish this, we displayed occurrence information at an HUC12 level and discussed the collection record at a waterbody level (creek, river, reservoir). We reported abundance as coarse abundance, and the following categories are used: Rare < 3, Present 3–25, Common 26–100, and Abundant > 100.

All collections followed the methods approved by the Animal Use and Care Committee at Pennsylvania State University (PROTO201800659). Surveys conducted by PFBC personnel followed agency fisheries sampling protocols for wadable and non-wadable streams, rivers, and lakes [51] but were modified when deemed necessary to target P. bimaculata. Modifications included focusing on P. bimaculata habitat, combining gear types to cover a location most effectively, and experimenting with electrofishing waveforms.

3. Results

Distribution in North America: The extent of the known historic distribution of P. bimaculata included the Chesapeake Bay watershed in the District of Columbia, Maryland, Pennsylvania, and Virginia. Limited sections of the lower Potomac and Susquehanna rivers, portions of their tributaries, and a few direct tributaries to the Chesapeake Bay were known to have been occupied [1,2,7,52].

Percina bimaculata is now extirpated from the Potomac River system, where historic records include the District of Columbia, Maryland, and Virginia (United States National Museum (USNM) 9731 and 68171) [7]. Smith and Bean [52] reported that “This large darter is not uncommon in gravelly streams of the District. It takes the hook freely and is sometimes caught by boys fishing from the city wharves”. Truitt et al. [53] stated that Uhler and Lugger reported it in Potomac tributaries south of Washington, the Chesapeake and Ohio Canal, and Pumunkey Creek. The extirpation of the P. bimaculata was likely the result of pollution and sedimentation [54,55].

In the Susquehanna River, it occupied at least 70 km (43 mi) from Havre de Grace, Maryland upriver to Columbia, Pennsylvania [1,2]. It has also been recorded from the following Susquehanna River tributaries: Broad, Conowingo, Deer, Mill, Octoraro, and Swan creeks in Maryland, Michael and Muddy runs, and Fishing, Peters, Muddy, and Octoraro (including the East and West branches) creeks, Pennsylvania. There are also older records from within or near the tidal limits of Winters Run (1949) and Northeast Creek (1950, 1996), which drain directly into the upper portion of the Chesapeake Bay in Maryland [8,19].

Historic Occurrence and Abundance in Pennsylvania (prior to 1950): The extent of occurrence includes only the Piedmont Province of the lower Susquehanna River drainage (Figure 3). Both Haldeman [1,2] accounts noted the locality to be the Susquehanna River. Fowler [56] and Near [8] examined the type specimen Academy of Natural Sciences Philadelphia (ANSP) 22652 for the Perca (Percina) nebulosa description [1]. Fowler [56] added Lancaster County (Figure 2) to the distribution of P. caprodes not included in Fowler [57]. Collette and Knapp [58] discussed a Percina type specimen, USNM 1405, attributed to Haldeman and stated it “may be the type of P. bimaculata, but we have been unable to find this specimen”. The online USNM, Fishes Collections specimen/lot information for the USNM 1405 record indicates the “precise locality” to be Susquehanna River, Columbia, Pennsylvania.

At the time of the Haldeman descriptions [1,2], the Susquehanna River at Columbia was recently impounded by the Columbia and Wrightsville Dam. Also, in the mid-1830s, Haldeman moved to a newly constructed residence located at Chickies, Lancaster County, Pennsylvania [59]. This location is at the mouth of Chiques (aka Chickies) Creek, approximately 2 km (1.6 mi) upriver from present-day Columbia and approximately 12 km (7.2 mi) downriver from his childhood home at Locust Grove. The lack of specificity regarding the collection locality in the Haldeman accounts [1,2] and the unknown origin of the addition of “Columbia” in the precise locality information associated with USNM 1405 make the exact type locality debatable. It is possible that the actual Haldeman collections, reported from the Susquehanna River, may have occurred above the Columbia and Wrightsville canal basin, in riffles present near the mouth of Chiques Creek where Haldeman resided, in the riffles below the Columbia and Wrightsville Dam, or possibly in the canal basin on the waterfront of Columbia.

No additional information has been found that describes the distribution or abundance for the latter half of the 19th or early 20th centuries. Bean [4], Fowler [5,56] and Thompson [60] listed the presence of P. caprodes in the Susquehanna River, Lancaster County without adding to the historic distributional understanding. It is inexplicable that Cope [61] and Cooper [62] did not discuss a population of P. caprodes from the Susquehanna River. The Haldeman records are the only historic information available for this species upriver of the present-day location of the Holtwood Dam; consequently, an understanding of the historic extent of distribution and abundance is only inferred (

Table 2. Occurrences and abundances by HUC 12 watershed for Percina bimaculata in Pennsylvania during the historic (prior to 1950), recent historic (1950–2000), and recent (post 2000) periods. Map identification number corresponds with HUC 12 watersheds in Figure 3, Figure 4 and Figure 5. LO = likely occurred. EX = extirpated.

Map ID#	HUC 12 Name	HUC 12 Code	Historic Events	Historic Abundance	Recent Historic Events	Recent Historic Abundance	Recent Historic Avg Abundance	Recent Events	Recent Abundance	Recent Avg Abundance
1	Susquehanna River—Conewago Creek	020503051011	LO		EX			EX
2	Conewago Creek (Dauphin/Lancaster Co.)	020503051010	LO		EX			EX
3	Fishing Creek (Conewago Ck., York Co.)	020503051009	LO		EX			EX
4	Susquehanna River—Chickies Creek	020503061702	LO		EX			EX
5	Conoy Creek	020503061701	LO		EX			EX
6	Codorus Creek	020503060707	LO		EX			EX
7	Donegal Creek	020503060803	LO		EX			EX
8	Little Chickies Creek	020503060802	LO		EX			EX
9	Chickies Creek	020503060804	LO		EX			EX
10	Susquehanna River—Fishing Creek	020503061704	2	2?	EX			EX
11	Kreutz Creek	020503061703	LO		EX			EX
12	Fishing Creek	020503061705	LO		EX			EX
13	Otter Creek	020503061707	LO		EX			EX
14	Susquehanna River—Pequea Creek	020503061706	LO		EX			EX
15	Conestoga River	020503061107	LO		EX			EX
16	Pequea Creek	020503061204	LO		EX			EX
17	Susquehanna River—Muddy Creek	020503061708	LO		1	1	1	14	108	8
18	Muddy Creek	020503061304	LO		1	1	1	8	302	38
19	Susquehanna River—Haines Branch	020503061709	LO		30	188	6	77	1091	14
20	Susquehanna River—Conowingo Dam	020503061712	LO		7	163	23	5	8	2
21	East Branch Octoraro Creek	020503061403	LO		LO			8	239	30
22	West Branch Octoraro Creek	020503061501	LO		LO			34	1618	48
23	Octoraro Creek	020503061502	LO		LO			12	28	2
	Totals		2	2?	39	353	8	158	3394	20

). It is noteworthy that historic collections would have largely predated electrofishing gear and relied mainly on nets.

Abbott [63,64] reported P. caprodes from the Delaware River and considered them “by no means abundant”. Fowler [65] noted that the material collected by Abbott was reviewed by Haldeman and he declared it identical to P. nebulosa. Fowler [66] also noted he had not collected P. caprodes from the Delaware River and subsequently did not include a population from the Delaware River in his Pennsylvanian works [5,56,57]. Fowler [66] later states it may have been introduced to the Delaware River. Mihursky [67] discussed the Abbott reports and considered them undoubtedly misidentifications. Whether the Abbott reports were the result of misidentification or an unsuccessful introduction, no additional reports have been found from the Delaware River drainage.

Recent Historic Occurrence and Abundance in Pennsylvania (1950–2000): The frequency of collection reports and surveys increased in the latter half of the twentieth century (Table 2). A fisheries study intending to inform fish passage issues at Conowingo Dam was conducted in 1957–1960, which included areas above and below the dam in Pennsylvania and Maryland. Percina caprodes was reported from seining surveys conducted below Conowingo Dam at the “Susquehanna Flats”, the head of the Chesapeake Bay below the mouth of the Susquehanna River [37]. Unfortunately, seining was not conducted as part of the assessment of Conowingo Pond [37]. The P. caprodes seine collections from the Flats in 1958 (n = 4) and 1960 (n = 1) are important in that they help begin to illustrate a contemporary distribution in the lower Susquehanna River, although they are not Pennsylvania records. The extent of its distribution recorded in Pennsylvania during the recent historic collection period included only Conowingo Pond and four tributaries (Figure 4).

Following the completion of the power generation facilities in the late 1950s and 1960s, aquatic assessment surveys on the lower Susquehanna River were conducted to monitor impacts from those operations on the biota of the river, including fishes. Near [8] listed 12 collections from Pennsylvania that are deposited at the Cornell University Museum of Vertebrates. These collections were made in 1966 and 1967 by Ichthyological Associates personnel using seines and trawls during monitoring surveys for the construction of the Muddy Run Pumped Storage Facility [68]. These are the first reports of P. bimaculata (as P. caprodes) available for Pennsylvania following those of Haldeman [1,2] and they predate the operation of migratory fish passage operations at Conowingo Dam (1972). Raney [68] reported collection localities to include the Susquehanna River and Fishing Creek in Lancaster County and Muddy Creek in York County. Ten of the 1966 to 1967 collections contain a single individual, indicating it was rare, while the others contain four and five, indicating it was present [68]. Ichthyological Associates continued surveys into the 1970s [69]. Robbins [70] reported collecting P. bimaculata with seines (n = 9), a trap net (n = 1), and in trawls (n = 15) during surveys conducted in Conowingo Pond and tributaries by Ichthyological Associates in 1974.

The Radiation Management Corporation, Ecological Division (RMC) reported P. bimaculata from Conowingo Pond during the period of 1967 to 1977 and 1979 while conducting monitoring surveys for the PBAPS [71,72]. Abundances of P. bimaculata were rare to present (n = 1–7) in seines and trawls during these efforts. Denoncourt and Cooper [73] summarized and reported on the fishes of the Susquehanna River above Conowingo Dam during this period also but did not add new information regarding P. bimaculata.

Tributaries to Conowingo Pond were occupied during this period as well. Biologists from the PFBC reported P. bimaculata as present from backpack electrofishing surveys of Fishing Creek, Lancaster County in 1978 and 1993 and Michael Run, York County in 1994; however, no abundance data were recorded. Staff from the PFBC reported P. bimaculata to be abundant (n > 100) in Fishing Creek during backpack electrofishing surveys in 1993. Personnel from PSU recorded occurrences of P. bimaculata from Peters Creek where it was common (n = 37), and Fishing Creek, where it was present (n = 3), in 1986 while searching for the Maryland Darter (Etheostoma sellare).

In 1997, Normandeau Associates, Inc. (Normandeau Associates) reported surveys at five sites in Conowingo Pond using seines, trawls, and boat electrofishing gear where abundances were rare to common (n = 1–45) [74]. In 1998, Normandeau Associates reported summarized data for surveys conducted on Conowingo Pond near PBAPS using seines, trawls, and boat electrofishing gear, totaling 71 individuals [75].

Recent Occurrence and Abundance in Pennsylvania (post 2000): Survey frequency and intensity in the 21st century increased dramatically with our efforts that specifically targeted P. bimaculata with specialized gears throughout its historic range (Table 2). The extent of distribution recorded in Pennsylvania during the recent collection period included Conowingo Pond and five of its tributaries, Octoraro Creek and its branches, Octoraro Reservoir, and the Muddy Run Pumped Storage Facility (Figure 5).

In 2004 and 2005, we initiated surveys in Conowingo Pond and tributaries to document the continued occupation of historic localities. During boat electrofishing surveys in 2004, we found populations at multiple locations within Conowingo Pond where it was rare to present (n = 1–25) and in Fishing Creek where it was rare (n = 1). During backpack electrofishing surveys in 2005, it was locally common in Fishing Creek (n = 29) and Peters Creek (n = 50).

In 2010, we conducted surveys throughout the lower Susquehanna River specifically targeting P. bimaculata using an electrified benthic trawl deployed from a jet-powered motorboat. A total of 45 trawl hauls was conducted on the Susquehanna River from York Haven Dam downriver to the Pennsylvania and Maryland border on Conowingo Pond. Sites were selected based on the suitability of the habitat for P. bimaculata, in particular, those not reachable by boats powered by propellers. These surveys were supplemented with available reports of surveys conducted within the historic distributional range of P. bimaculata [76,77,78,79]. At the conclusion of these efforts, it was determined P. bimaculata was, without question, extirpated from its historic range upriver of Holtwood Dam and the PFBC subsequently listed it as state-threatened [15]. We found four localities occupied within Conowingo Pond. Three localities were occupied during the recent historic period, and the fourth was a new locality. Abundances at these localities indicate P. bimaculata was rare to present (n = 2–5). The fourth was a new locality directly below Holtwood Dam on the right descending bank, an area that receives less consistent flow than the left descending bank, where the hydroelectric facility operates. We are not aware of additional collections in this area. Similar to our efforts, Normandeau Associates conducted seine and boat electrofishing surveys on Conowingo Pond in 2010. Normandeau Associates reported P. bimaculata collections from 19 sites in Pennsylvania [80]. These sites were mainly along shorelines and well distributed throughout Conowingo Pond. Abundances were also rare to present (n = 1–9) per survey site.

Collections on Conowingo Pond from 2011 to 2023 covered much of this section of the Susquehanna River and historically occupied tributaries. From 2011 to 2016, nine collections were reported from Conowingo Pond by Normandeau Associates, PFBC, and SRBC personnel with abundances being rare to common (n = 1–79). From 2018 to 2023, our efforts combined with those of the PFBC totaled 17 surveys conducted on Conowingo Pond with abundances ranging from rare to common (n = 1–64). We visited Muddy Creek eight times from 2018 to 2023 and documented abundances to be present to abundant (n = 3–213). Our efforts combined with those of the PFBC during 2015 to 2020 totaled 12 surveys on Fishing Creek with abundances ranging from rare to common (n = 2–26). Our efforts combined with those of the Lancaster Conservation District from 2018 to 2023 totaled 25 surveys on Peters Creek with abundances ranging from rare to abundant (n = 1–135). Our efforts combined with those of the PFBC during 2016 and 2021 totaled two surveys on Michael Run with abundances ranging from rare to present (n = 1–3).

In 2016, PFBC personnel documented P. bimaculata to be present (n = 3) in Muddy Run, upstream of the Pumped Storage and Recreation lakes during a backpack electrofishing survey. In a similar survey, we documented it to be rare (n = 1) at this site again in 2020. In 2021, we documented a previously unknown population in the Muddy Run Recreation Lake at three locations. Abundances were rare to present at collection sites (n = 1–4).

During towed barge electrofishing surveys in 2007, the first specimens of P. bimaculata were collected from Octoraro Creek, Lancaster County by Normandeau Associates, where it was present (n = 6) at two adjacent sites [81]. Electrofishing surveys by the Academy of Natural Sciences (R. Horwitz) in Tweed Creek and below Octoraro Dam found populations to be present (n = 4) and rare (n = 1) in 2008. During a backpack electrofishing survey in 2008, we found it to be rare (n = 1) at a site between the Maryland state line and Octoraro Dam. Also in 2008, we found it to be locally common in short sections of the east branch (n = 27) and west branch (n = 52) of Octoraro Creek above the dam. During a tow barge survey between the Maryland state line and Octoraro Dam in 2009, we found it to be present (n = 5). From 2012 to 2014, both Normandeau Associates and PFBC personnel reported five additional collections in the Octoraro Creek drainage that corroborated previous surveys with similar abundances (n = 1–4).

In 2020, we surveyed Octoraro Reservoir with a boat electrofisher and found five locations within the impoundment to be occupied, a body of water previously not known to have populations of P. bimaculata. Abundances within Octoraro Reservoir ranged from rare to present (n = 1–24). We also documented four tributaries to the reservoir that were occupied, including Gable Run, Leach Run, and the west branch and east branch of Octoraro Creek. Populations in Gable Run and Leach Run are represented by only one collection in each stream, and abundances were rare (n = 1–2). Many surveys were conducted with seines, backpack electrofishers, and boat electrofishers on the West Branch and East Branch of Octoraro Creek from 2017 to 2023 supporting reintroduction efforts (Table 2), which included collections of brood stock for hatcheries and wild fish for translocations. Abundances ranged from rare to abundant (n = 1–207) in the West Branch and present to abundant (n = 13–103) in the East Branch. Fluctuations in abundances were noted in the West Branch during this time period and potentially explained by a drawdown of Octoraro Reservoir in 2020 and multiple extreme precipitation events.

4. Discussion

The lower Susquehanna River was and still is an important center for agriculture, industry, transportation, and power generation. Correspondingly, it has had a complicated history of pollution and habitat impacts. Pollution and sedimentation have been identified as the probable causes for the extirpation of the Potomac River population [54], and it is likely the same is true for the Susquehanna River population from Columbia to Holtwood. At this time, it is not possible to precisely determine how or when P. bimaculata was extirpated from the river above Holtwood given the paucity of available information; however, many factors known to be detrimental to fishes were documented to have affected the Susquehanna River for many decades in the 19th and 20th centuries. Most significantly, the lower Susquehanna River was affected by coal silt and related mining activities, municipal and industrial pollution, agricultural run-off, and the construction of impassible dams on the river and its tributaries.

The absence of P. bimaculata from all survey efforts upstream of the present location of Holtwood Dam since Haldeman’s last collection [2] indicates it has been extirpated from at least 30 km (19 mi) of the Susquehanna River. The lowermost sections of tributaries in this section of river were probably also occupied historically based on what has been observed in Conowingo Pond. This is a reduction in its range in the Susquehanna River of approximately at least 32%. It is also likely that this species occurred upriver as far as Conewago Falls, the present site of York Haven Dam. Percina bimaculata is currently known to occupy approximately 20 km (12 mi) of the Cononwingo Pond of the Susquehanna River and limited sections of five tributaries: Muddy Creek, Muddy Run, Fishing Creek, Peters Creek, and Michael Run. The Muddy Run Recreation Lake is occupied, and the Muddy Run Pumped Storage reservoir is likely occupied. Approximately 35.5 km (22 mi) of Octoraro Creek and its east and west branches are occupied. Octoraro Reservoir is also occupied, adding approximately 9 km (6 mi) of streams, all impounded.

Recent historic (1950–2000) and recent (post 2000) surveys on Conowingo Pond have demonstrated that P. bimaculata survived the habitat conversion of the Susquehanna River below Holtwood Dam from that of the Conejohela Valley to an impoundment (above Conowingo Dam) or riverine habitats influenced by modified flow through an upriver dam (e.g., Holtwood Dam). While the lower Susquehanna River impoundments unquestionably reduced its preferred habitat, the conversion did not eliminate its useable habitat.

In Conowingo Pond, tributaries have been demonstrated to be important, evidenced by the consistent collections in the lower sections of those tributaries and in the river within the vicinity of their mouths. Abundance in recent (post 2000) surveys varies widely from rare to abundant. These streams are generally in good condition and contain ample clean sand, gravel, rubble, and boulder substrates. Some smaller tributaries not occupied by P. bimaculata similarly contribute clean sand and gravel to the river, and collections have occurred from the river at their mouths. It is possible that P. bimaculata utilized unimpacted tributaries or minimally impacted sections of the river as refugia during periods of poor water quality. It seems reasonable to suggest that conditions below Holtwood and Conowingo dams and/or in tributaries remained suitable for P. bimaculata through the period in which it was extirpated from the section of river above Holtwood. A lack of clean tributaries or the lack of access to clean tributaries (i.e., the presence of a mill dam or waterfall) would heavily influence the ability of P. bimaculata to persist in an impoundment with little preferred habitat and a depressed water quality. There are few tributaries present in the section of the Susquehanna River from Columbia-Wrightsville to Holtwood that could have served as refugia, and those that are present would have been barriered or likely would have had similar water quality issues (e.g., Conestoga River, Pequea Creek).

Similar to Conowingo Pond, we found Octoraro Reservoir to be occupied by P. bimaculata at multiple sites where a suitable habitat was found, including a few tributaries and tributary mouths. Until 2008, all PFBC surveys conducted on the branches above Octoraro Dam (1976–1977, 1989–1990, 2006) were performed upstream of the sites where populations of P. bimaculata were first documented in 2007 and 2008. In 1967 and 1968, Edwin L. Cooper surveyed Octoraro Creek below the dam and in both branches without encountering P. bimaculata, which is not easily explained (E. L. Cooper, unpublished field notes). Personnel from the PFC surveyed below the dam in 1986 and did not encounter them. Despite the lack of historic reports, it is likely populations were present in Octoraro Creek and its East and West branches and went undetected prior to the creation of the impoundment in 1952. The adaptability of P. bimaculata to lacustrine conditions then allowed it to colonize suitable habitats within the impoundment. All known collections from below Octoraro Dam are post 2007. The lack of collection reports below the dam could have multiple explanations. This section of Octoraro Creek was seldom surveyed, and it was, until recently, bookended by Octoraro Dam and a smaller dam near the mouth in Maryland. The lower dam was removed in 2005 [82] and now permits access to the Pennsylvanian section of the stream from the Susquehanna River. Recent survey data indicate that P. bimaculata is present to abundant in the East and West branches, rare to present in Octoraro Reservoir, and rare to present in Octoraro Creek from Octoraro Dam to the Maryland border. Additional surveys need to be undertaken in Octoraro Reservoir to adequately assess its abundance and the full extent of its occupation. We believe populations are likely more abundant in Octoraro Reservoir than the cursory surveys we conducted in 2020 would indicate.

Our discovery of a population in Muddy Run Recreation Lake is not surprising given the demonstrated utilization of suitable habitats within Conowingo Pond and Octoraro Reservoir; however, the means of its arrival there is likely novel. Muddy Run has a small drainage area, and prior to the construction of the Muddy Run Pumped Storage Facility, P. bimaculata would not have been expected from anywhere other than near its mouth. A 1958 stream assessment report for Muddy Run by PFC staff describes a stream likely too small for P. bimaculata [83]. The lower section of Muddy Run would have been drastically reorganized by the Pumped Storage Dam and its intakes and impoundments. The means by which P. bimaculata colonized the Muddy Run drainage likely resulted from the pumping of water from the Susquehanna River at the intake structures, where P. bimaculata is present both in recent historic and recent surveys, to the Muddy Run Pumped Storage Reservoir. Migration from the Pumped Storage Reservoir to the upper Recreation Lake likely occurred through their shared connection. Another explanation is possible: fishes captured from the Susquehanna River might have been stocked in the Recreation Lake, but we have seen no documentation to support this. Recent survey data indicate that P. bimaculata is rare to present in the Muddy Run drainage. The colonization of the Muddy Run drainage is a rare instance of a relatively recent range extension by P. bimaculata and demonstrates the variety of environments that can be potentially occupied.

Historic collections were limited to net gear types that primarily focused on shallow, wadable environments. These early investigations were limited in extent and number. During the recent historic collection period, the need for monitoring in relation to the construction and operation of power generation facilities increased efforts, but these initially utilized net gear types (seines, trawls, and trap nets). Eventually, electrofishing gears (boat and backpack) became more commonly used. In the recent collection period, both monitoring surveys, similar to those conducted in the recent historic period, and targeted surveys were undertaken by us and others. Targeted surveys used the most appropriate gear for the collection locality and conditions (seines, backpack electrofishers, towed barge electrofishers, boat electrofishers, and electrified benthic trawls). We have noted that the multitude of gear types we used in the recent collection period resulted in no one gear type being much better suited for collecting P. bimaculata. In many cases, our surveys using an electrified benthic trawl in the Susquehanna River collected similar samples in similar localities as boat electrofishers; however, the electrified benthic trawl captured P. bimaculata in some locations that would not have been conducive to boat or backpack electrofishing. The highest abundances were documented using electrofishing gears.

Efforts in the recent collection period greatly increased the number of surveys conducted and the number of P. bimaculata encountered (Table 2), but the extent of the distribution did not increase along the Susquehanna River. In Conowingo Pond, the same number of HUC 12 watersheds were occupied in the recent period as the recent historic period, a total of four (Figure 4 and Figure 5; Table 2). Available survey information indicates that these populations in Conowingo Pond and tributaries are at least stable. The addition of three HUC 12 watersheds in the Octoraro Creek drainage by our survey efforts in the recent period is best explained as an artifact of few historic sampling efforts and possibly the effects of dams at multiple locations until 2005. Some surveys indicate that P. bimaculata can be locally common or abundant, especially those targeting them, but most surveys document less than 10 individuals at a locality.

Habitat Utilization: We believe that P. bimaculata is primarily a large-river form, and the riverine metapopulation is supported by additional sub-populations moving between the Susquehanna River (or impoundment, i.e., Conowingo Pond and Octoraro Reservoir) and suitable tributary streams. We are uncertain of the precise relationship between river and tributary but suspect that P. bimaculata cannot survive in smaller streams alone. When P. bimaculata became extirpated from the Potomac River, it also disappeared from the tributaries reported on by Smith and Bean [52] and Truitt et al. [53]. Similarly, it is not extant in Susquehanna River tributaries in the Holtwood or Safe Harbor impoundments. With the loss of much of the free-flowing habitat in the mainstem river, these tributaries may assume greater importance for spawning and refugia than during the pre-impoundment era.

Distributions in tributaries to the Susquehanna River are further limited by barriers that occur along the Fall Zone (high gradients or waterfalls) on their lower courses. Muddy, Fishing, and Peters creeks (tributary to Conowingo Pond) all have a pronounced Fall Line that defines the upper limit of occupation by P. bimaculata. In Octoraro Creek, the only tributary to the lower Susquehanna River extending into Pennsylvania that does not have a defined Fall Line, the upper headwaters would have naturally been accessible to fishes from the river. We collected P. bimaculata from stations on the East Branch and West Branch of Octoraro Creek where the mean width was 18.9 m and 15.3 m, respectively, but it was absent at upstream stations where the mean widths were 14.0 m and 13.1 m, respectively. We are not aware of any Pennsylvania collections from smaller sections of waterways other than those above, except the aforementioned tributaries associated with Conowingo Pond and Octoraro Creek.

During surveys in wadable waters, P. bimaculata was captured adjacent to larger, coarse substrates like boulders, rubble, and large woody debris. In the spring and early summer, individuals, pairs, or schools of P. bimaculata were often observed and/or captured over clean sand and gravel, unassociated with large, coarse substrates. In Conowingo Pond, Susquehanna River, similar habitats were generally present when P. bimaculata was collected at tributary mouths, islands, and shorelines, but observing the position of fish in relation to substrates was not possible, especially when using benthic trawls. We have also observed young-of-year fish and juveniles occupying the same locations as adults.

We found P. bimaculata to occupy three reservoirs in Pennsylvania, including Conowingo Pond, Octoraro Reservoir, and Muddy Run Recreation Lake. Populations of P. bimaculata occurring in reservoirs mimic the natural use of lacustrine systems by P. caprodes and its adaptability to impoundments [60,84,85,86]. It is likely the Muddy Run Pumped Storage Impoundment is also occupied at times, given the presence of P. bimaculata near its intake structures in the Susquehanna River and the populations present above it in Muddy Run Recreation Lake. The Pumped Storage Impoundment is a novel environment given the radical swings created by the operation of the facility for electric power generation.

Conservation and Threats: The demonstrated tolerance of impoundments by P. bimaculata suggests that reintroduction efforts above Holtwood and Safe Harbor dams have the potential to reestablish populations mimicking those of Conowingo Pond, Muddy Run Recreation Lake, and Octoraro Reservoir. Dam removals on tributaries to the Susquehanna River above Holtwood have reunited the river with additional kilometers of suitable habitats composed of sand, gravel, rubble, and boulder substrates. For example, multiple impassable dams on Chiques Creek were recently removed, including one near its mouth (Heistand Sawmill Dam) in 2015. Pequea Creek was once dammed for an early hydroelectric operation near its mouth and is now open to the Susquehanna River again. Recovery within the zone of extirpation (York Haven to Holtwood) appears to be plausible, but challenges still exist there and to extant populations in Conowingo Pond, its tributaries, and Octoraro Creek.

Impingement of the logperch has been reported [71] at the Peach Bottom Atomic Power Station and still occurs [87]. Entrainment almost certainly occurs at the Muddy Run Pumped Storage Facility. If P. bimaculata was to recolonize the Susquehanna River above Holtwood, impingement at cooling water intakes associated with Brunner Island Steam Electric Station would also be expected.

Depauperate native fish assemblages such as that of the Susquehanna River Basin have been characterized as vulnerable to non-native invasions [88]. Introduced fish species richness has been generally related to large-scale environmental disturbances [89]. Hocutt et al. [13] and Snyder [90] reported a rather large portion of the Susquehanna River’s total fish diversity to be composed of introduced species, 28% and 31%, respectively, which rank among the highest within the central Appalachians and central Atlantic Coastal Plain drainages [13]. We found approximately 46% of the species (11 of 24) collected during the 2010 survey of the lower Susquehanna River to be introduced.

Competition for space and resources with introduced species of similar habitat preferences and feeding guilds may be a significant interspecific threat to P. bimaculata. Of particular concern are the introductions of the Banded Darter (Etheostoma zonale) [49,62] and Greenside Darter (E. blennioides) [49] to the Susquehanna basin, presumably in the 1960s. The Susquehanna River Basin has a depauperate native darter fauna consisting of five species, one of which is rare (P. bimaculata), and another is extirpated (E. sellare, Maryland Darter) [13,49,91,92], which may predispose the basin to the widespread colonization of non-native darter species. Similar colonization after introduction by the Mimic Shiner (Paranotropis volucellus) [62] should also be noted in this discussion. All three of these species were widespread and collected in relatively large numbers during our recent surveys. Etheostoma blennioides, E. zonale, and P. volucellus were also present in Octoraro Creek during recent surveys.

In the spring of 2020, 21 Northern Snakeheads (Channa argus) were provided passage from below Conowingo Dam to Conowingo Pond via the east fish lift [93]. Subsequently, the PFBC received multiple reports from the public of C. argus being caught by anglers on Conowingo Pond. In July 2023, we surveyed throughout Conowingo Pond with boat electrofishing gear as part of monitoring efforts and found C. argus to have established a reproducing population and to have expanded in the extent of its range; however, abundances were rare to present. Also, during these surveys, we found P. bimaculata to be present to common in abundance, and C. argus was present at every location at which P. bimaculata was encountered, indicating an overlap of summer habitats in Conowingo Pond. It is currently unclear if and to what degree C. argus poses a threat to P. bimaculata.

Percina bimaculata is likely utilized in the diet of many piscivorous predators. It has been documented in the stomachs of White Crappie (Pomoxis annularis) in Conowingo Pond [94]. The following non-native species occurrences are also noteworthy in the lower Susquehanna River. The voracious and fast-growing Flathead Catfish (Pylodictis olivaris) was found at many localities and in multiple length classes on the lower Susquehanna River throughout our 2010–2023 surveys. Blue Catfish (Ictalurus furcatus) are known from below Conowingo Dam [95]. During SCUBA monitoring surveys conducted by PSU, P. bimaculata cultured in a hatchery and reintroduced to Chiques Creek were witnessed to have been preyed upon by Rusty Crawfish (Faxonius rusticus).

We suggest that P. bimaculata is a recoverable species in Pennsylvania. While it is unlikely any of the large dams affecting P. bimaculata will be removed in the foreseeable future, other conservation actions can be accomplished resulting in positive impacts. Projects that maintain or increase water quality in the lower Susquehanna River drainage would benefit P. bimaculata. Ensuring that the dissolved oxygen in impoundments does not become depressed can expand or maintain populations in these habitats. Programs that limit the transport of eroded soils and the further sedimentation of preferred habitats should benefit P. bimaculata. Limiting the introduction and expansion of non-native species that compete with or prey upon P. bimaculata would reduce unwanted interactions. A long-term and prudent approach to reintroduction efforts (see introduction section) would allow mangers to take full advantage of the potential for P. bimaculata to utilize both permanently modified habitats and preferred habitats made available once again by environmental and conservation efforts.